A user equipment uses a random access channel (RACH) to access a network in a state that the user equipment is not uplink synchronized with a base station. A signal having repetitive characteristic in a time domain is used in the random access channel, so that a receiver easily searches a start position of a transmission signal. In general, the repetitive characteristic is realized by repetitive transmission of a preamble.
A representative example of a sequence for realizing the preamble includes a CAZAC (Constant Amplitude Zero Auto Correlation) sequence. The CAZAC sequence is expressed by a Dirac-Delta function in case of auto-correlation and has a constant value in case of cross-correlation. In this respect, it has been estimated that the CAZAC sequence has excellent transmission characteristics. However, the CAZAC sequence has limitation in that maximum N-1 number of sequences can be used for a sequence having a length of N. For this reason, a method for increasing available bits of the sequence while maintaining the excellent transmission characteristics is required.
Meanwhile, there are provided various methods for transmitting data from a random access channel by using the CAZAC sequence. Of them, the first method is to directly interpret CAZAC sequence ID to message information. Assuming that data to be transmitted is a preamble, if a sufficient number of sequences that can be used as the preamble are provided, message passing can be performed with only CAZAC sequence ID without additional manipulation. However, since a method of transmitting additional information should be considered in an actual synchronized RACH, problems occur in that there is difficulty in realizing a sufficient number of CAZAC sequence sets, and the cost required for search of a receiver increases.
The second method is to simultaneously transmit CAZAC sequence and Walsh sequence by using a code division multiplexing (CDM) mode. In this case, CAZAC sequence ID is used as user equipment identification information, and the Walsh sequence transmitted in the CDM mode is interpreted as message information. FIG. 1 is a block schematic view illustrating a transmitter for realizing the second method. However, the second method has limitation in that even though the Walsh sequence is added to the CAZAC sequence, bits of message that can additionally be obtained are only log2N bits when the Walsh sequence has a length of N.
The third method is to transmit CAZAC sequence and Walsh sequence in such a way to mix the Walsh sequence with the CAZAC sequence. In this case, CAZAC sequence ID is used as user equipment identification information, and the Walsh sequence is interpreted as message information. FIG. 2 is a block diagram illustrating a data processing procedure at a transmitter for realizing the third method. However, according to the third method, since the Walsh sequence acts as noise in detection of the CAZAC sequence to cause difficulty in detecting sequence ID, there is limitation in that repetitive sequences should be transmitted to prevent the Walsh sequence from acting as noise in detection of the CAZAC sequence.
The fourth method is to either give orthogonality between blocks constituting a corresponding sequence by multiplying an exponential term by a CAZAC sequence or directly apply data modulation such as DPSK, DQPSK, D8PSK, etc. In this case, CAZAC sequence ID is used as user equipment identification information, and the modulated sequence is demodulated and then used as message information. FIG. 3A illustrates data modulation according to the former method of the fourth method, and FIG. 3B illustrates data modulation according to the latter method of the fourth.
Furthermore, the fifth method is to transmit CAZAC sequence by attaching a message part to the CAZAC sequence. FIG. 4A illustrates the case where a message (coded bit) is attached to the CAZAC sequence used as a preamble, and FIG. 4B illustrates the case where a message (coded bit) is attached to a sequence consisting of a predetermined number of blocks to which orthogonality is given.
However, the fourth method and the fifth method have a problem in that they are susceptible to change of channel condition.